1. Field of the Invention
This invention relates to the production of block copolymers prepared by rapid crosslinking of miscible mixtures of two or more homopolymers. More specifically it relates to the production of block copolymers, such as blocks of high-vinyl-polybutadiene and of low-vinyl-polybutadiene, by the irradiation of a miscible mixture of two or more homopolymers. Still more specifically, it relates to the production of such block copolymers while maintaining the copolymers below or slightly above the gellation state.
2. Related Prior Art
When a mixture of two or more different homopolymers of type A, B, etc. is crosslinked or coupled, two possibilities arise. If the homopolymers are immiscible, crosslinking will take place independently in the phase-separated regions composed of homopolymer molecules A and in the regions composed of homopolymer molecules B, etc. That is, mostly A-A and B-B crosslinks will be formed because homopolymer molecules A will almost exclusively be surrounded by homopolymer A molecules and, conversely, homopolymer B molecules will be predominantly surrounded by B molecules. There will be a few A molecules in contact with B molecules along the interface of the domains of this heterogeneous blend. This should lead to some A-B or copolymer links, but the volume contribution of such links should be extremely small considering the small number of molecules located in the interface regions. The result of such crosslinking will therefore be a heterogeneous mixture of higher molecular weight homopolymers with eventual gelation of one or the other or both polymers occurring when the number of crosslinks introduced exceeds the critical crosslink density required for gelation.
As used herein the term "miscible" means that the components of a blend or mixture are completely compatible or homogeneous at the temperature being used. Miscibility of two or more polymers depends on a number of factors including similarity of structural units in the respective polymers, molecular weights of the polymers, temperature of the mixture, proportions of the respective components, etc. For example, two particular polymers may be compatible at a particular molecular weight range and incompatible or immiscible at higher molecular weights. Moreover, two particular polymers may be immiscible at room temperature but may become miscible at higher temperatures.
If the polymer components are totally miscible, hompolymer molecules A will find themselves surrounded by both molecules A and molecules of polymer B, etc. Then, provided that the structure of the two types of molecules permit crosslinking by one mechanism or another, crosslinking or coupling should become possible not only between AA and BB molecules, but also between A and B molecules. If the crosslinking is continued, it will be likely that molecules which have already undergone one crosslinking step will continue to crosslink. For example, a molecule which has been formed by the crosslinking of an A and a B molecule (now referred to as AB molecule) may undergo further crosslinking with either an A molecule or a B molecule or an AA molecule or a BB molecule or another AB molecule forming ABA, ABB, ABAA, ABBB or ABAB structures, respectively. The resulting molecule now composed of three or four original units can undergo further crosslinking resulting in longer and even more complicated structures. The sequence of events described for the case of a miscible blend of molecules A and B could very easily be extended to miscible blends made up of molecules of types A, B, C. etc.
Depending on the structure of the homopolymers, it may be that while A and B molecules of a certain molecular weight are miscible, similar polymer molecules of a higher molecular weight such as are formed by AA and BB linking are not miscible anymore. The incorporation of B molecules in A molecules such as is accomplished through an A-B link will tend to make AA and BB molecules more compatible, but even then a point may come where partial or complete phase separation will occur. This phase separation will require a certain period of time governed by the molecular weight of the polymers involved, the temperature, the presence of diluents, etc. Because molecules in such a system contain segments of homopolymer structure A and B linked together, such polymers are called block copolymers. These materials are useful as compatibilizing agents but they also can find utility as product by themselves as adhesives, coatings, etc.
In the course of making these copolymers, problems may be encountered because a gelation of the miscible blend may be induced on prolonged crosslinking. The utility of a gelled material as a compatibilizing agent, etc. would be very limited. At the gel point, few, very long molecules, because of the greater number per molecule of crosslinking sites, will start to form a network while a large fraction of the low molecular weight molecules of types A and B have undergone little, if any, crosslinking. In the terminology of polymer science, that means that the weight average molecular weight is approaching infinity at the gel point while the number average molecular weight of the mixture has increased only slightly. The reason for this phenomenon is that the probability for a molecule to crosslink with another molecule depends on its molecular weight - the higher the molecular weight, the greater the number of crosslinking sites and the greater the probability for crosslinking. The further increase in molecular weight through crosslinking of long chain molecules is thus more favorable in contrast to the less favorable growth and crosslinking of the shorter chain building blocks A and B. As it is the object to link as many A and B molecules as possible to form the polymer of the desired structure, it is obvious that an early gelation of the mixture is undesirable since, as stated before, many of the homopolymer molecules A and B have at that point not yet been linked together in any form. It is obviously undesirable therefore that gelation of any portion of the mixture should occur prior to crosslinking or coupling of substantially all of the A and B homopolymer molecules. In other words, there is a greater tendency for the larger polymer molecules to grow to the gelation stage than there is for the low molecular weight homopolymers A and B to join together.